The invention relates to syringe drivers and systems for effecting a controlled infusion of a medical fluid from a syringe.
Certain medical fluids are administered by controlled parenteral infusion and require a slow but non-rate critical flow. Infusion of a medical fluid in this manner has been generally accomplished by use of a drip bag gravity-feed system or an electronic infusion pump. The drip bag provides non-rate critical flow with a simple and relatively inexpensive apparatus.
In certain applications, particularly those involving small fluid volumes, the use of disposable syringes is preferred to drip bags. The disposable syringe is a commodity item. Syringes may be prepared in large quantities and stored for later use, thereby reducing costs and providing greater reliability and repeatability of preparation. Also, a single syringe may be used to provide multiple doses, simply by advancing the syringe plunger in fixed increments. For example, it may be desired to fill a 60 cc syringe with 40 cc of fluid and to then administer the fluid in 10 cc doses, with a flow rate such that a single dose is delivered in twenty to forty minutes. Doses may then be repeated at fixed intervals, with four doses being delivered from a single disposable syringe.
There exist several advantages to delivering multiple doses from a single dose delivery system. A single large syringe is less expensive than several small syringes or other single dose devices. Significant time savings can be realized in filling, handling, and administering the syringes since the overall number of operations is greatly reduced. There may also be a decrease in patient risk due to the fact that the intravenous delivery system must be opened fewer times for component changes, and so there are fewer opportunities for contamination or handling errors.
Administration of medical fluids at a low flow rate using a syringe is generally accomplished by the use of a syringe pump. A typical syringe pump is a motorized, programmable device on which a syringe is mounted, and which expels fluid from the syringe by advancing the syringe plunger at a controlled rate. The motorized pump advances the syringe plunger at a predetermined velocity, applying force as required to maintain this velocity. The flow rate is therefore independent of fluid resistance unless the resistance becomes so high that the pump cannot safely provide the corresponding force required. The advantages of using disposable syringes for non-rate critical applications are diminished by the cost and complexity of existing syringe pumps. Typical pumps are battery powered, microprocessor controlled, provide numerous programming options and annunciators, and require detailed operator training and ongoing maintenance and inspection. Much of the cost and complexity of the syringe pump is directed to the requirements of rate critical applications and is not required for non-rate critical applications. See, U.S. Pat. Nos. 4,804,368 (Skakoon); 4,544,369 (Skakoon); and 4,943,279 (Samiotes).
Several devices have been developed that are intended to provide less complex and expensive means of providing non-rate critical infusion with a disposable syringe. One option is to replace the electronic components of a typical infusion pump with a spring driven clockwork mechanism. While this type of device has been described, no actual devices are known to have been commercialized. See, U.S. Pat. Nos. 4,059,110 (Wuthrich) and 4,676,122 (Szabo).
A second option is based the use of a predetermined force, rather than a predetermined velocity, that is applied to the syringe plunger so that fluid resistance acts to control the flow rate. This force may be applied by a simple mechanism, such as a weight, or compressed or extended spring of appropriate stiffness. See, U.S. Pat. No. 4,132,231 (Puccio). By appropriate selection of applied force and fluid resistance, the resulting flow rate can be engineered to a desired level. Fluid resistance may be controlled by varying the length and internal diameter of the tubing that connects the syringe to the patient. Appropriate tubing for this application is known as precision microbore tubing because it has a significantly smaller bore diameter than standard tubing used with drip bags or motorized pumps, and because it is manufactured to defined tolerances and specific end points having to do with resistance to fluid flow.
A commercial device based on this alternative is the "Medifuse" device available from 3M Healthcare. In this device, a "Neg'ator" rolled spring is used to apply an approximately constant force to a syringe plunger. This is combined with a selection of microbore tubing sizes, each with a different internal diameter to provide control of flow rate. A somewhat complicated roller and track mechanism is required to counteract the applied torque that is a consequence of the use of the "Neg'ator" spring. The "Medifuse" device functions as intended but its applicability is limited because it does not provide control of multiple doses from a single syringe. The "Neg'ator" spring, because of its rolled design, is susceptible to changes in spring force due to contamination by sticky, abrasive, or particulate material. Moreover, a spring of this type may be easily damaged by improper handling during cleaning. An additional limitation is that the activating mechanism for initiating a dose on the "Medifuse" device involves pivoting a large cover component, which can be clumsy and which requires additional clearance. Also, a syringe cannot be mounted within a "Medifuse" device without initiating fluid delivery from the syringe. U.S. Pat. Nos. 4,202,333 (Thill); 4,298,000 (Thill); 4,430,079 (Thill); and 4,597,754 (Thill).
Another commercial device, the "Band-It" from I-Flow, Inc., described in U.S. Pat. No. 5,429,607 (McPhee), employs microbore tubing but applies force to the syringe plunger using an elastomeric element, such as a stretched band of latex rubber. This device is extremely simple but, again, does not enable delivery of multiple doses. Also, it does not provide a constant or nearly constant flow rate because the force applied by the elastomeric element varies significantly over the range of plunger travel. This device further requires manual stretching of the elastomeric element and provides no mechanism to assist in this action. Like the "Medifuse" device, a syringe cannot be mounted in a "Band-It" unit without causing fluid to be delivered from the syringe.
Other syringe driver systems and described in U.S. Pat. Nos. 4,636,197 (Chu); 4,608,042 (Vanderveen); and 5,318,539 (O'Neil).
Known mechanical, predetermined force-applying devices are not well suited to the delivery of multiple doses from a single syringe. Conversion of these devices to achieve multiple dose control might be accomplished using removable stops to limit the range of plunger travel. By sequentially removing the stops, the plunger could move in a series of fixed increments. Such stops, however, are not intrinsically safe. It is possible to remove several stops at the same time, or to remove the stops in the wrong order. The result can be an overinfusion condition involving the delivery of two or more doses in a continuous manner.
Another drawback of such devices is that the force applied to the plunger can decrease over the range of travel of the plunger. The result is that successive doses will be delivered from the same syringe at increasingly lower flow rates. Various additional references that describe dose setting include U.S. Pat. Nos. 2,632,445 (Kas); 4,498,904 (Turner); 5,232,459 (Hjertman); 5,300,041 (Haber); 5,328,486 (Woodruff); and 5,092,842 (Bechtold).